1. Field of the Invention
The present invention relates to a method of modifying a driving voltage to the transparency curve (V-T curve) of a liquid crystal display (LCD). In particular, the present invention relates to a method of modifying the V-T curve of the LCD by modulating voltage waveform on the common electrode.
2. Description of the Related Art
The V-T curve of a LCD is usually determined by its operating liquid crystal mode (hereafter called LC mode). Usually the V-T curve of the LCD is fixed and cannot be changed.
FIG. 1 (PRIOR ART) shows an example of the V-T curve of the LCD. There is a threshold voltage Vth in the V-T curve. If the driving voltage is lower than the threshold voltage Vth, the LC molecules can not incur any reorientation. Under this situation, the change of the driving voltage cannot adjust the optical transparency rate T of the LCD. In addition, the LCD cannot be operated when the driving voltage is lower than the threshold voltage Vth.
In FIG. 1, the range of the effective operating voltage is between voltage Vth and voltage V0.
It is known that the output voltage of a data driver applied in a typical LCD is limited. Therefore, the higher the threshold voltage Vth, the narrower the effective operating voltage range of the data driver. In addition, it shrinks the physical voltage range with respect to the various gray levels. Accordingly, it is difficult to divide gray levels and the number of effective gray levels of the LCD decreases. For example, when uniformly dividing 1.5V effective operating voltage range into 8 bits (256 levels) of gray levels, the average interval between two gray levels is only 5.86 mV. If the output noise of the data driving IC is 10 mV, the LCD can merely display 128 gray levels corresponding to 7 bits or lower.
Under a fixed output noise level, therefore, to obtain better performance of effective gray levels by the LCD, we must enlarge the effective operating voltage range of the data driving IC.
To solve the problem found in the prior art, the present invention provides a method of modifying the V-T curve of LCD. By changing the voltage waveform of the common electrode, it is possible to control the V-T curve of the LCD and the value of the threshold voltage (Vth). By this method, the range of the effective working voltage of the data driving IC in the LCD can be enlarged.
Accordingly, the present invention provides a method of modifying the V-T curve of the LCD. This LCD has a plurality of pixels. Each pixel has liquid crystal and a transistor. The drain terminal of the transistor is connected to a data line. The gate terminal of the transistor is connected to a scan line. The source terminal of the transistor is further connected to a first electrode of one side of the liquid crystal having a second electrode at the other side connecting to a common electrode. A characteristic of the present invention is provision of a changed voltage waveform to the common electrode to modify the V-T curve of the LCD such that the threshold voltage of a data driving IC is reduced and the range of a working voltage is increased.
Furthermore, the method of this present invention mainly changes the voltage waveform of the common electrode during the front porch and the back porch of a driving sequence of the LCD. For example, the amplitude of the changed voltage waveform is in the same or inverse phase and smaller than the amplitude of a positive and negative square wave of a data voltage waveform. Besides, the amplitude of the changed voltage waveform is in the same or inverse phase and greater than the amplitude of a positive and negative square wave of a data voltage waveform.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.